Three different types of optical phenomena have been observed at high altitude above thunderstorms: an enhanced airglow ("elves") at roughly ---90 km; a reddish glow ("sprites") from 50 to 90 km; and an upward moving, bluish emission ("jets") below 40 km. A likely explanation for some or all of these phenomena is gas breakdown caused by the electromagnetic fields of lightning discharges. This paper examines the connection between these fields and breakdown at high altitude, using both analytic models and numerical simulations. Included in the calculations are the radiation fields from the lightning return stroke and the quasi-static fields from the continuing current. The different nature of the two fields is shown to produce two distinct types of breakdown, with characteristics similar to those of elves and sprites. Also mentioned is a third breakdown mechanism which may account for blue jets.
29,65329,654 FERNSLER AND ROWLAND: LIGHTNING-PRODUCED SPRITES AND ELVES
Large lightning discharges can drive electromagnetic pulses that cause breakdown of the neutral atmosphere between 80 and 95 km leading to order of magnitude increases in the plasma density. The increase in the plasma density leads to increased reflection and absorption, and limits the pulse strength that propagates higher into the ionosphere.
Electromagnetic pulses (EMP) driven by lightning can cause breakdown of the neutral atmosphere in the lower D‐region. Using a computer simulation model, we study the dependence of the breakdown on the pulse strength, the orientation of the lightning discharge, the ambient plasma density, the ionization model, and the neutral density. For a discharge along a straight line the EMP is strongest in the plane perpendicular to the current so that for a given current, horizontal discharges will radiate the D‐region more strongly than a vertical discharge. For horizontal currents, breakdown occurs for E100 > 20 V/m (I > 55 kA) in a low‐density, nighttime ionosphere, where E100 is the amplitude of the pulse normalized to 100 km from the discharge and I is the discharge current. Vertical strokes require E100 > 50 V/m (I > 140 kA). Discharges with higher currents and fields form ionization patches which are larger in volume, larger in degree of ionization, and lower in altitude. The ionization is most sensitive to the pulse strength, pulse orientation, ambient plasma density, and neutral gas density at breakdown threshold. Higher ambient plasma densities reduce the ionization, but for large EMP, breakdown can occur even with high daytime densities. The breakdown increases the plasma density which acts to limit the EMP and ionization. This feedback reduces the sensitivity of the breakdown to the ionization model. Neutral density variations, such as caused by atmospheric gravity waves, can cause spatial variations in the ionization density.
A series of computer simulations is reported showing the generation of electromagnetic radiation by strong Langmuir turbulence. The simulations were carried out with a fully electromagnetic 2 1/2 -dimensional fluid code. The radiation process takes place in two stages that reflect the evolution of the electrostatic turbulence. During the first stage while the electrostatic turbulence is evolving from an initial linear wave packet into a planar soliton, the radiation is primarily at ωe. During the second stage when transverse instabilities lead to the collapse and dissipation of the solitons, 2ωe and ωe radiation are comparable, and 3ωe is also present. The radiation power at ω=2ωe is in good agreement with theoretical predictions for electromagnetic emissions by collapsing solitons.
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